Cadmium Zinc Telluride detectors for a next-generation hard X-ray telescope

https://doi.org/10.1016/j.astropartphys.2021.102563Get rights and content

Abstract

We are currently developing Cadmium Zinc Telluride (CZT) detectors for a next-generation space-borne hard X-ray telescope which can follow up on the highly successful NuSTAR (Nuclear Spectroscopic Telescope Array) mission. Since the launch of NuSTAR in 2012, there have been major advances in the area of X-ray mirrors, and state-of-the-art X-ray mirrors can improve on NuSTAR’s angular resolution of 1arcmin Half Power Diameter (HPD) to 15” or even 5” HPD. Consequently, the size of the detector pixels must be reduced to match this resolution. This paper presents detailed simulations of relatively thin (1mm thick) CZT detectors with hexagonal pixels at a next-neighbor distance of 150 μm. The simulations account for the non-negligible spatial extent of the deposition of the energy of the incident photon, and include detailed modeling of the spreading of the free charge carriers as they move toward the detector electrodes. We discuss methods to reconstruct the energies of the incident photons, and the locations where the photons hit the detector. We show that the charge recorded in the brightest pixel and six adjacent pixels suffices to obtain excellent energy and spatial resolutions. The simulation results are being used to guide the design of a hybrid application-specific integrated circuit (ASIC)-CZT detector package.

Introduction

Cadmium Zinc Telluride (CZT) detectors are an attractive detector technology for hard X-ray astronomy as they offer excellent spatial resolutions, good energy resolutions, and, compared to Si and Ge detectors, much larger photoelectric effect cross sections at hard X-ray energies. A CZT imager may be used on a next-generation telescope succeeding the space-based hard X-ray telescope NuSTAR [1]. The high stoppping power and excellent energy resolution of the NuSTAR CZT detectors enabled it to image the Cassiopeia A (Cas A) supernova remnant in the 67.9 keV and 78.4 keV line emissions from the radioactve isotope 44Ti [2], [3], [4]. Contrary to the soft X-ray lines detected previously, the nuclear 44Ti emission directly tracks the yield of nuclear material independent of the temperature and density of the ejecta [2], [3], [4].

The recently developed monocrystalline silicon X-ray mirrors [5] or electro-formed-nickel replicated (ENR) X-ray optics [6] promise angular resolutions with Half Power Diameters (HPD) of between a few arcseconds and 15 arcseconds – even at hard X-ray energies. The proposed HEX-P [7] and BEST [8] observatories seek to capitalize on this technology, as the point source sensitivity scales linearly with the angular resolution. Nyquist sampling the images provided by the improved X-ray mirrors requires detectors with excellent spatial resolutions. Our group is thus leading the development of new small-pixel CZT detectors with center-to-center pitch of 150 microns and hexagonal pixels, improving by a factor of four over NuSTAR’s CZT detectors (605-micron pixel pitch).

This paper discusses the simulations performed for the design of the third-generation Hyperspectral Energy-resolving X-ray Imaging Detector [HEXID3 [9], [10]], which features hexagonal pixels at a next-neighbor pitch of 150 μm and uses a low noise front end design achieving a projected readout noise of 14 electrons Root Mean Square (RMS). The advantage of using hexagonal over square pixels is that all the nearest neighbors of any given pixel are equivalent; in square pixels, some immediate neighbors are closer than others. Another similar ASIC for hybridization with pixelated CZT detectors is the High Energy X-ray Imaging Technology (HEXITEC) ASIC developed by Rutherford Appleton Laboratory. The HEXITEC ASIC features 6400 square pixels at a next-neighbor pitch of 250 μm with an electronic readout noise of 50 electrons RMS [11], [12], [13].

Our simulations model in detail the interactions of the incident photons, secondary photons and high-energy electrons generated in the CZT, and the ionization losses of the latter. The simulations furthermore model the drift and diffusion of the negative and positive charge carriers through the CZT, including the effects of mutual repulsion of charge carriers of equal polarity. This detailed treatment allows us to predict the properties of the signals, including the pixel multiplicity, and the dependence of the pixel signals on where in the detector the free charge carriers are generated. Earlier discussions of CZT detector simulations can be found in [13], [14], [15], [16], [17]. Compared to the earlier study of small pixel detectors of [13], the shape of our charge clouds evolve owing to charge carrier repulsion and diffusion as the clouds drift inside the detector. Furthermore, we extend the study from a pixel pitch of 250 μm to smaller 150 μm pixels.

The rest of the paper is organized as follows. After describing the detector simulation methodology in Section 2, we present the results of the simulations in Section 3. Our studies show that the 1mm thick detectors have a limited energy range over which they give excellent performance. We discuss the results and implications for the camera of a NuSTAR follow-up mission in Section 4.

Section snippets

Simulations of the CZT/ASIC Hybrid Detectors

X-rays impinging on a CZT detector interact via photoelectric, scattering, and pair production interactions. Photoelectric interactions dominate up to primary photon energies of Eγ<240keV at which Compton scattering becomes dominant (assuming 40% Cd, 10% Zn and 50% Te). The photo-electron of a photoelectric effect interaction loses most of its energy to ionization. The ionization promotes electrons to the conduction band, generating clouds of electrons and holes. Applying a bias across the

Results

In this section, we discuss methods to reconstruct the energy of the incident photon, and the location of the primary interaction. We will first discuss the results obtained in the absence of readout noise, and the show how they change as we add the noise expected for the HEXID ASIC.

Discussion

In this paper, we present simulations of 1 mm thick CZT detectors with hexagonal pixels at an extremely small pixel pitch of 150 μm. The detector simulations account for the spatially distributed generation of free charge carriers in the detector, and the drift and diffusion of the charge of both polarities. The simulations furthermore account for the anticipated charge resolution of the HEXID3 ASIC. We have shown that the sum of the signals of the brightest pixel and the adjacent pixels and

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We thank Grzegorz Deptuch, Gabriella Carini, and Shaorui Li for their work on the HEXID ASIC, as well as the McDonnell Center for the Space Sciences at Washington University in St. Louis for its support. We thank Richard Bose and Andrew West for designing a HEXID readout system and HEXID photomasks. HK acknowledges NASA support under grants 80NSSC18K0264 and NNX16AC42G.

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